1. Technical Field
The present invention relates to the general field of Faraday rotators and, more particularly, to enhancing the performance of Faraday rotators including reducing drive current and improving latching capability.
2. Description of Related Art
Certain materials exhibit the property of rotating the plane of polarization of plane polarized light passing through the material when an external magnetic field is applied. This effect is known as the Faraday effect and we denote materials that exhibit the Faraday effect as xe2x80x9cmagneto-optic materials.xe2x80x9d Devices making use of applied magnetic fields to rotate the plane of polarization are known as Faraday rotators. Faraday rotators have been employed in components or subsystems in various optical devices including optical switches, isolators, circulators, attenuators, among other devices. The importance of such devices will increase as communication networks make increasing use of light as the means of communication.
Magneto-optic materials used in conventional Faraday rotators have several limitations. For example, some magneto-optic materials are incapable of switching. Such non-switching Faraday rotators can be used in isolators and circulators but lack the full range of applications of a switchable Faraday rotator. On the other hand, many switchable magneto-optic materials are not xe2x80x9clatchable,xe2x80x9d or possess only poor latching capability. xe2x80x9cLatchingxe2x80x9d denotes the ability to retain sufficient magnetic field on the magneto-optic material when the applied external magnetic field is removed such that the functionality of the Faraday rotator is not seriously degraded. One objective of the present invention relates to providing a Faraday rotator having switching functionality with latching capability.
However, merely achieving switching capability and latching capability in a Faraday rotator may not be sufficient to produce a practical commercial device. Other performance characteristics of a Faraday rotator are also desirable (or essential in some applications). For example, it is important in many applications that the magnetic field required to effect switching not be too large. Some magneto-optic materials typically used for latchable Faraday rotators can require quite large switching fields (e.g. possibly exceeding 1000 Oersteds (xe2x80x9cOexe2x80x9d)). In general, for magneto-optic materials to be useful in practical switching applications, it is preferable to have a low coercive field Hc such that switching does not require high current and power. Present Faraday rotators often require too much driving power to permit such switching Faraday rotators to be attractive components for use in most practical optical systems. The dual difficulties of lack of stable latching capability and the need for high switching current has precluded magneto-optic (Faraday rotator) switches from being widely used in optical switches.
In addition to the general difficulties of combining latching and low power switching discussed above, some magneto-optic materials may not be capable of performing the switching operation over the entire range of temperatures to which a commercial device may be subject (e.g. approximately 0xc2x0 C. to approximately 65xc2x0 C.). Achieving a Faraday rotator that is both switchable at reasonable field strengths and capable of latching behavior is one objective of the present invention.
FIG. 1 depicts a conventional prior art Faraday rotator in a schematic, cut-away view (not drawn to scale), such as disclosed in the U.S. Pat. No. 4,609,257 to Shirasaki. The device of FIG. 1 includes a magneto-optic material 1, electromagnet including a current-carrying coil 3 generating a magnetic field, and a semi-hard magnetic material 2. In the operation of the Faraday rotator of FIG. 1, the electromagnet applies a magnetic field, H, to magneto-optic material, 1. Current flowing through coil 3 from left to right in FIG. 1 generates magnetic field H in the direction shown by the arrow H. To change the rotation direction of the Faraday rotator, the magnetic field is reversed by reversing the current in coil 3 causing a change in the direction of magnetization in the electromagnet, 2, to Hxe2x80x2 as shown by the broken arrow in FIG. 1.
However, the Faraday rotator depicted in FIG. 1 has several drawbacks, including the following: Since the coil, 3, encircles only portion of the magnet (i.e., Part A as shown in FIG. 1), the maximum magnetic field is delivered only to the material surrounded by coil 3. The part that closes to the magneto-optic material 1 (i.e., Part B as shown in FIG. 1) is significantly less strongly magnetized due to the distance L separating the magneto-optic material from the coil 3. Leakage of magnetic field occurs as the field traverses the space from A to B in FIG. 1. Since the magnetic reluctance of the magnetic circuit is proportional to the length of the path (i.e., path L as shown in FIG. 1) magnetic flux at Part A cannot be totally transmitted to the Faraday rotator 1 since the permeability of the intervening material is not infinite (as discussed in standard references including David Jiles, Introduction to Magnetism and Magnetic Materials, 2nd Ed. (Chapman and Hall, London), pages 54-57 (1998)). Thus, part of the magnetic energy generated by coil 3 is wasted. It follows that switching requires the application of sufficient current to coil 3 to produce the field strength necessary for switching as well as allowing for leakage between A and B. Reducing the current required for switching is one objective of the present invention.
The work of Shirasaki et. al. U.S. Pat. No. 5,812,304 adds a second source of magnetic field to the coil depicted in FIG. 1. The magnetic field generated by the second magnetic unit is perpendicular to the magnetic field generated by the first magnetic unit (i.e. the coil) in order to produce a more uniform magnetic field in the magneto-optic materials. However, in terms of magnetic field leakage, this work has the same limitations as those discussed in connection with FIG. 1.
Takeda et. al. disclose in U.S. Pat. No. 5,048,937 a Faraday rotator depicted schematically in FIG. 2. The Takeda device consists of (a) magneto-optic material 4, (b) a wire coil 5 encircling the magneto-optic material for the purpose of changing the magnetization state of the Faraday rotator, and (c) a hollow yoke 6 surrounding the assembly of coil and magneto-optic material. Again, the coil 5 does not encircling the hollow yoke 6 pursuant to this disclosure. We note in connection with the Takeda reference that the maximal magnetic field of the solenoid coil is inside the coil (i.e., position C as depicted in FIG. 2). Since the hollow yoke is located outside of the coil, the magnetic field at the position of the hollow yoke (i.e., position D) generated by the coil is much smaller than the magnetic field at position C within the coil. Thus, this device has the disadvantage of not effectively magnetizing the hollow yoke.
FIG. 3 depicts a Faraday rotator as disclosed Shirai et. al. in U.S. Pat. No. 5,535,046. In this design, a portion of a magnetic garnet film is exposed to a localized magnetic field. As depicted in FIG. 3, magnetic fields directed in antiparallel directions are applied in different localized portions of the magnetic garnet film by permanent magnets. A coil provides a magnetic field in addition to the field generated by the permanent magnets for the purpose of altering the total field applied to the magneto-optic material. The coil is not depicted in FIG. 3.
FIG. 3 depicts the upper localized magnetic field as directed in the positive x-axis direction, while the lower localized magnetic field is directed in the negative x-axis direction. The field applied to the magneto-optic material by the external coil lies in either the positive or negative x direction. Therefore, there is always a localized magnetic field that is directed in the direction opposite to that of the applied external magnetic field. Under these conditions the single domain structure of the magnetic field within the magneto-optic material cannot be maintained when the external applied magnetic intensity is removed. Thus, the design depicted in FIG. 3 functions mainly to remove the latching effect of magnetic garnet. However, this is contrary to one of the objectives of the present invention of enhancing the latching capability of the Faraday rotator. The present invention discloses a technique distinct to that of FIG. 3 for enhancing latching.
The present invention relates to the use of magnetic materials in combination with magneto-optic materials to improve performance of Faraday rotator devices by enhancing the latching capability and reducing the driving current. Semi-hard magnetic materials are advantageously used in connection with some embodiments of the present invention. Placing such semi-hard magnetic materials in proximity with the magneto-optic material enhances latching and reduces the drive current required to cause switching. Encompassing both the magneto-optic material and the semi-hard magnetic material further reduces drive current requirements and enhances latching by reducing leakage of the magnetic field from its generation within the coil to the magneto-optic material. Some embodiments further include soft or semi-hard material encompassing the coil. Semi-hard magnetic materials having coercivities in the range from approximately 20 to approximately 100 Oe are advantageously used.